No Power to Offside Side Light

[wiredsoftware]

Hi all,

 

I got a warning light on the dash the other day for bulbs. The front O/S side light turned out to be the problem. However, replacing the bulb hasn't fixed it, and after checking the wiring diagram there appears to be no power to the grey/black wire that supplies the bulb at the connector.

 

I don't suppose this is a known issue, but if anyone can point me at any obvious places to check before I take out the battery, airbox and everything else in that side of the engine to chase the wire?

 

Apparently it comes from the "on-board power supply control unit" (any idea where that is?), and the N/S light is fed on a separate wire from there (which presumably is why the N/S is working fine). 

 

Car is a 2005 Octavia 1.9 TDI.

 

If I can't find the control unit, or the unit turns out to be defective, presumably I can just run power from the N/S? Not sure how the warning light knows that it is gone though, would a bulb warning on the dash be an MOT failure if all lights worked?

[Papfox]

If you put a meter on it with the engine and lights off then turn the ignition on but don't start the car, do you get 4 short pulses of voltage on the lamp cable?

 

My car BCM tests the bulb like that at ignition-on and, if it doesn't detect a valid bulb, shuts the lamp down. I had the same symptom as you when I first tried experimenting fitting LEDs to my car. If you see the test pulses and then the line goes dead it could well be that BCM thinks there's a problem with the bulb and has shut it down.

[Breezy_Pete]

I'm guessing you mean nearside (left when sitting in car) judging by the wire colour you mention?

Onboard supply control unit is approximately above the accelerator pedal, I think.

 

Not 100% sure but I'd be quite surprised if a bulb warning was an MOT failure if all the bulbs work.

[Papfox]

I believe for an MoT pass all warning lights must be out.

 

I would never encourage such behaviour but I did once hear of a friend who removed the warning light bulb to get through an MoT in such circumstances.

[wiredsoftware]

Yeah sorry got my N/S and O/S reversed in the post, you're correct! That's what happens when you work on the car parked on the wrong side of the road

 

Interesting about the pulses, will check again when I buy some fresh bulbs - tested with an old standard bulb I had lying around and a LED one, so will try a new bulb to be 100% sure and check the pulses are present. 

 

Are the pulses long long enough to pick up on a digital multimeter? Don't have access to an analogue one at the moment. 

 

 

Edited 2 April, 2017 by wiredsoftware

[Papfox]

I haven't tried it. The pulses are very short so a meter probably won't get to 12V but I imagine you should see something that's not zero when you turn the ignition on.

 

Are your LEDs the "CANBus" ones? Those are made to take more current so the car thinks it's an ordinary bulb in the socket. When I first tried Halfords 501 LED bulbs in mine, I experienced exactly what you're reporting. The car didn't even try to light them.

 

Edited 2 April, 2017 by Papfox

[wiredsoftware]

All sorted now, guess the regular

bulb I tried must have been broken, and the LED one didn't work I guess. Anyway, once I put another bulb in, removed the key and left it for a few minutes, it started working again...

[hrvrs]

I think the car ECU will cut the power to a sidelight bulb when it detects a "fault", so if you turn on the ignition with no bulb and connect the meter afterwards it may not show much